1. Field of the Invention
This invention relates generally to the molding of objects and more particularly to the formation of molding tools using powder metal technology and more particularly to inclusion of thermal control elements in those tools.
2. Description of Related Art
Molding techniques are used to form many types of objects from many types of materials. These techniques have a common element of shaping the material to form the object by contacting the material with a molding surface, wherein the shape of the molding surface is transferred to the object material. This molding surface is a surface of a tool to be used in the molding process. At the time of shaping the object material is, in whole or in part, typically in a flowable state. Before separating the molding surface and the object material, the object surface transitions into a solid or at least a semi-solid state whereby it is no longer significantly flowable. This transformation may occur as a result of the surface of the object material cooling below its freezing point or solidus point. This process of using a reduction in temperature to cause the object material to transition between a first phase (e.g., flowable or liquid phase) to a second phase (e.g. solid phase) may be referred to as a thermal phase change molding process. Alternatively, this transition might occur by the object material undergoing a chemical reaction, to a sufficient extent, to cause it to solidify. As a further alternative, the transition might take place by a combination of these two techniques.
For these molding techniques to work, the molding surface must have appropriate properties, such as retaining its shape during the molding process and being separable from the object material. In some molding processes the molds are used only once then destroyed (e.g. investment or lost wax casting). In still further processes the tool is intended to retain its shape through the molding of tens, thousands, or even millions of copies of the object (e.g. injection molding and die casting).
As the latter process noted above involves the use of the same tool to successively produce copies of an object, the time it takes to form each copy can become a critical factor in the molding process. As such, it is typically desired that the molding time per copy (i.e. the cycle time) be as small as possible. Furthermore, as many such processes involve thermal phase changes to achieve the molding process, the time required to cool the material from its flowable state to its solid state is a critical factor in determining cycle time. The more heat conductive the tool and the more heat that can be removed from the tool per unit time, the faster the cycle time can be.
As such, molds for many fabrication methods are typically cooled to decrease the cycle times. Examples are plastic injection molding and die casting, where molten material is injected into the mold and the material must cool and solidify before it is ejected. The cycle time is the duration of time to inject the material, cool and solidify the material, open the mold, eject the part, and close the mold and be ready for the next injection. Currently mold inserts (i.e. the tool portion that includes the molding surface) and bases (i.e. an additional portion of the tool that holds the insert in position) are drilled, bored, or otherwise machined to produce passages for coolant flow (e.g. water or oil) or passages for insertion of other temperature control elements (e.g. heating elements). In particular when these passages are used for coolant flow, the method has allowed the heat from repeated molding cycles to be removed and has greatly reduced molding cycle times and speeded up fabrication of molded parts.
In addition to the need for cooling to improve cycle time, there are at least three other reasons why in some circumstances it might be desirable to control mold temperature by cooling and/or heating. Some materials and processes benefit from use of temperature control so as to minimize distortion of the copy of the object. It is also typical to control the temperature of the molding surface by heating and/or cooling to achieve acceptable surface finish. It is also typical that the mold surface be operated at a desired temperature and with a particular heat dissipation rate such that the flowable material will appropriately be distributed in the mold cavity prior to transition.
The typical process of machining passages into the tool is difficult, time consuming, costly and not always as effective as possible at minimizing cycle times and as effective as possible at producing objects with desired levels of accuracy. It is recognized, by those of skill in the art that the current methods of controlling the tool's thermal characteristics are not optimal. These thermal characteristics include passive characteristics such as thermal conductivity, convection, and radiative attributes of the tool shape and material. Some tools may have active thermal characteristics as well. These active thermal characteristics include such things as fluid flow within the tool and supplemental heating. Furthermore, some tools may have enhanced thermal characteristics as well. These characteristics may result from use of materials with increased thermal conductivity such as cooling pipes and the like.
The typical method of drilling passages does not typically provide the fastest cycle times possible, and it does not typically provide optimal active or enhanced thermal control across the face of the mold. In some circumstances optimal thermal control may involve active, enhanced, and/or passive uniform removal of heat from the molding face of the tool. In other circumstances, optimal thermal control might involve active, enhanced, and/or passive non-uniform heat removal from the surface. For example, if the surface element is near a large body of molding material, more heat may need to be extracted from that element than if the element were in proximity to a small volume of molding material. Optimal thermal control (i.e. temperature control and rate of heat removal) is an important requirement when uniform physical properties of the resulting molded material are important.
Typically the closer the cooling passages conform to the shape of the molding surface (ideally known as conformal cooling), the more uniform the temperature control and the rate of heat removal.
Also, as noted above, typical production of these passages has other disadvantages. The tools, especially the inserts, are typically made from very hard metal such as tool steel, thus making drilling or otherwise forming the passages difficult, costly, and time consuming.
Furthermore, these passage production operations produce straight flow paths within single pieces of the tool and may produce some step-wise curved paths where multiple pieces are brought together. This production of straight paths makes attainment of conformal cooling difficult, if not impossible.
A need exists in the art for easier and less costly production of cooling passages in tools. A further need exists for production of more optimal tooling, especially for tooling with more optimized control of mold temperature and heat flow within a tool.
Tooling production is described in "Plastic Injection Molding . . . manufacturing process fundamentals", by Douglas M. Bryce, which was published in 1996 as Volume 1 of a series entitled "Fundamentals of Injection Molding" by the Society of Manufacturing Engineers, Dearborn, Michigan. The entire disclosure of this publication is incorporated herein by reference as if set forth in full herein. In particular, this publication discloses main components of a molding machine on pages 11-27. Parameters associated with the molding process are addressed on pages 29-66. In particular, temperature control issues are addressed on pages 30-37. Optimization of mold parameters is addressed on pages 67-120. In particular, optimization of temperature is addressed on pages 78-93 wherein, among other things, cooling channels, cascades, and cooling pins are discussed. Pages 139-150 address basic issues associated with mold operation and design. Pages 195-218 address testing and failure analysis of the molded objects. Pages 219-253 address trouble shooting techniques for molding problems.
Production and use of tooling formed using powder metal technology are known in the art. Examples of techniques for such production and use are disclosed in the following U.S. Patents that are each herein incorporated by reference in their entirety:
(1) U.S. Pat. No. 3,823,002, entitled "Precision Molded Refractory Articles," issued July 1974 to Kirby et al.
(2) U.S. Pat. No. 3,929,476, entitled "Precision Molded Refractory Articles and Method of Making," issued December 1975 to Kirby et al.
(3) U.S. Pat. No. 4,327,156, entitled "Infiltrated Powdered Metal Composites Article," issued April 1982 to Dillon et al.
(4) U.S. Pat. No. 4,373,127, entitled "EDM Electrodes," issued February 1983 to Hasket et al.
(5) U.S. Pat. No. 4,432,449, entitled "Infiltrated Molded Articles of Spherical Non-Refractory Metal Powders," issued February 1984 to Dillon et al.
(6) U.S. Pat. No. 4,455,354, entitled "Dimensionally-Controlled Cobalt Containing Precision Molded Metal Article," issued June 1984 to Dillon et al.
(7) U.S. Pat. No. 4,469,654, entitled "EDM Electrodes," issued September 1984 to Hasket et al.
(8) U.S. Pat. No. 4,491,558, entitled "Austenitic Manganese Steel Containing Composite Article," issued January 1985, to Gardner.
(9) U.S. Pat. No. 4,554,218, entitled "Infiltrated Powdered Metal Composite Article," issued November 1985, to Gardener et al.
(10) U.S. Pat. No. 5,507,336, entitled "Method of Constructing Fully Dense Metal Molds and Parts," issued to Tobin.
The process of forming articles using powder metal technology, as described in the above patents, typically starts with a pattern of the tooling component (e.g. mold insert). This pattern may be formed with stereolithography or some other method. Then a temporary RTV (flexible material produced through a room temperature vulcanizing process) mold is cast around the pattern and the pattern is removed. Next a mixture of high melting point powder (e.g. metal and/or ceramic powder) and a binder is poured into the RTV mold, the binder hardens, and then the resulting "green" part is removed from the RTV mold. The green part is then heated in a furnace to remove the binder and to sinter the metal powder so that it is strong enough to handle. Typically the sintered component is not 100% dense. It is next heated in another furnace cycle where it is infiltrated with copper or another lower melting temperature metal to achieve full density and required mechanical properties. This mold insert has the desirable property that the infiltrated copper gives it higher thermal conductivity than an insert that is composed of 100% high melting temperature metal (e.g. A6 Tool Steel). Typically mold inserts formed with powder metal technology are drilled for cooling passages, as described previously.
Stereolithography and other Rapid Prototyping and Manufacturing technologies are described directly in the following U.S. patents and applications or indirectly through references incorporated therein by reference. The following listed patents and patent applications are fully incorporated herein by reference as if set forth in full:
U.S. Pat. No. 4,575,330, to Hull, describes some fundamental elements of stereolithography.
U.S. Pat. No. 5,321,622, to Snead et al., describes various techniques for manipulating three-dimensional object data to produce cross-sectional data for use in forming three-dimensional objects.
U.S. patent application Ser. No. 08/722,335 by Thayer et al. filed Sep. 27, 19996 now abandoned, and U.S. Pat. No. 5,943,235 to Ear. et al., respectively describe various issues related to the production of three-dimensional objects according to the principles of selective deposition modeling.
U.S. Pat. No. 5,965,079 to Manners, describes various techniques for solidifying layers of material sometimes known as build styles.
U.S. Pat. No. 5,902,538 by Kruger et al., discloses simplified exposure and coating techniques for forming 3D objects to overcome minimum coating depth limitations. Furthermore, this patent describes various RP&M technologies that can be used in the production of three-dimensional objects and supplies basic patent information associated with these various technologies.
The various RP&M technologies and some associated applications are described in the following two books which are incorporated by reference as if set forth in full herein: (1) Rapid Prototyping and Manufacturing: Fundamentals of Stereolithography, by Paul Jacobs, published by the Society of Manufacturing Engineers, Dearborn, Mich.; 1992; and (2) Stereolithography and other RP&M Technologies: from Rapid Prototyping to Rapid Tooling; by Paul Jacobs; published by the Society of Manufacturing Engineers, Dearborn, Mich.; 1996.